(a) Field of the Invention
The present invention relates to a gel for the separation of useful components from a rhodanate-containing detoxication treated liquid and to a method of separation using said gel.
(b) Description of the Prior Art
Hydrogen cyanide and hydrogen sulfide which are regarded as a cause of air pollution and water contamination are most often contained in fuel gases such as coke oven gas and cracked crude oil gas or in various unpurified raw gases. Therefore, in order to remove such toxic substances from these gases, various desulfurization and decyanization methods were proposed. Among others, the detoxication treatment described in "Kagaku Kogyo" (Chemical Industry) vol. 23, no. 7, p 922-927 (1972) is regarded as the most practical. This method is a combination of the Fumacks desulfurization method and Rhodacks decyanization method. The former is a wet type desulfurization method of sulfur recovery system in which hydrogen sulfide in the gas is separated and removed as sulfur by using as a catalyst, an aromatic polynitro compound such as picric acid, nitrophenol, trinitrobenezoic acid, nitro compounds of resorcin, etc. or a quinone type catalyst such as sodium naphthoquinone sulfonate, sodium anthraquinone sulfonate, etc. The latter is a decyanization method in which HCN, which is very toxic, is fixed and removed as a non-toxic rhodanate by washing the hydrogen cyanide-containing gas with an alkaline sulfur suspension. By this combination, the hydrogen cyanide and hydrogen sulfide in the gas are removed at high efficiency. In this detoxication method, the treatment liquid taken out of the reaction system contains, besides the rhodanate, an oxyacid salt of inorganic sulfur as a by-product. Furthermore, the above-mentioned aromatic polynitro compound as a catalyst and its reduction product are also contained in the treatment liquid, and therefore the liquid assumes a deep red color. Accordingly, it is not desirable to discharge it into rivers. Rather, the rhodanate and oxyacid salt of sulfur contained in the treated liquid are useful as industrial chemicals. Therefore, it is desirable to separate the useful components in the liquid and to utilize them for their respective uses. As a method of separating the useful components, i.e. the rhodanate and oxyacid salt of sulfur, the Applicant proposed a method which was described in Japanese Patent Publication No. 4000/78.
The cross-linked dextran gel used in this method is obtained by cross-linking dextran dissolved in water with epichlorohydrin in a reversed phase suspension system. As apparent from its production method and physical properties, the gel is a "soft gel" and has an inferior mechanical strength. Therefore, the gel, when used in a large scale industrial apparatus, can not display its ability sufficiently because of its destruction and compression. This necessarily creates a difficulty in elevating the treating ability, since, for example, there is a limit in increasing the rate of liquid passage. Furthermore, for elevating the separation ability, it is generally effective to use a gel of small particle diameters, but the use of such a gel results in increasing the operation pressure. This obliges us to use a gel of large diameters which gives a small pressure loss. After all, the use of the cross-linked dextran gel gives unsatisfactory separation results, and furthermore the concentrations of the useful components in the eluate must become low, which entails an economical disadvantage. In addition, in the operation on an industrial scale, since it is difficult to completely prevent suspended particles from coming into the treatment liquid, the suspended particles clog the packed gel layer, and this inevitably increases the operation pressure. When using the cross-linked dextran gel, which is mechanically weak, the back wash operation (operation to remove the blockage due to the suspended particles by back washing the packed gel layer) must be conducted frequently, and this substantially shortens the working time.
In the aqueous solution containing the remaining components after the recovery of the rhodanate, there are contained various catalyst components useful in the desulfurization and decyanization processes, and alkaline substances. The discard or loss of such useful components is not desirable for the operation of the desulfurization and decyanization processes from the industrial viewpoint.